(1) Field of the Invention
This invention relates to a method for producing (aryl substituted)carboxylic acid or its salt which is useful as a medicine or an intermediate for preparing organic chemicals such as medicines. More particularly, this invention relates to a method for producing highly pure (aryl substituted)carboxylic acid or its salt in which (aryl substituted)aldehyde is oxidized by using an oxidizing agent of hypohalogenite to form (aryl substituted)carboxylic acid and the reaction product is then refined by the hydrogen treatment of halogenated by-products contained in the aimed product of (aryl substituted)carboxylic acid.
(2) Description of the Prior Art
In connection with the method for preparing the (aryl substituted)carboxylic acid by oxidizing (aryl substituted)aldehyde, various oxidizing agents have been hitherto proposed. Among them, the methods using hypohalogenous acid or its salts as oxidizing agents, are regarded as preferable ones because their oxidizing efficiency is high. Methods of this kind are disclosed in the following references:
(a) In Japanese Laid-Open Patent Publication No. 53-18534 is disclosed a method for synthesizing 2-(p-isobutylphenyl)propionic acid as a medicine by oxidizing 2-(p-isobutylphenyl)propionaldehyde with hypohalogenite in the presence of acetic acid.
(b) In Japanese Laid-Open Patent Publication No. 55-2614 is disclosed a method for obtaining 2-(m-benzoylphenyl)propionic acid from 2-(m-arylphenyl)propionaldehyde by the oxidation with hypohalogenous acid. In a similar method disclosed in Japanese Laid-Open Patent Publication No. 56-113736, hypochlorite is used.
The present inventors have found the fact that the oxidation using hypohalogenite (hereinafter referred to as "oxidizing agent") excels in the oxidation efficiency but the generation of halogen molecules from the oxidizing agent cannot be avoided. In addition, it was also found that (aryl substituted)carboxylic acid is inevitably contaminated with halogenated by-products. The formation of these halogenated by-products is specific to the oxidation with this oxidizing agent, and yet, the analysis such as structural analysis of them is difficult owing to the diversity of them. The (aryl substituted)carboxylic acid is used in the fields in which high purity and high safety are required. In such the fields, the contamination with halogenated by-products even though its quantity is quite small, is not desirable for the product of (aryl substituted)carboxylic acid.
In order to eliminate impurities, recrystallization is generally adopted. However, it is well known that an aimed product also comes into the remained filtrate after the separation of precipitated crystals. The contamination with impurities like the halogenated by-products must be severely avoided, therefore, the quantity of an aimed product remained in the filtrate is increased on purpose to rise purity, which fact inevitably reduces the recovery rate of an aimed product.